Two-dimensional materials, such as graphene and transition metal dichalcogenides (TMDs), are a promising class of nanomaterials for next generation electronics, photovoltaics, electrocatalysts, sensors, and optoelectronic devices. TMDs are layered materials where the bonding between layers is weak van der Waals forces, resulting in the ability to separate the layers and study the properties of individual monolayers. MoS2, an exemplary TMD, is a layered semiconductor with a bandgap in the range of 1.2-1.8 eV and the physical properties of this material are thickness-dependent. For example, photoluminescense (PL) has been observed in this material as the material thickness is decreased—particularly in monolayers.
Various techniques are available for generating thin layers of MoS2. For instance, chemical vapor deposition (CVD) has proven to be a powerful tool for generating large-area, high quality, monolayer molybdenum disulfide. This technique has allowed for the generation of proof-of-principle devices and sensors that take advantage of the electronic and photonic properties of transition metal dichalcogenide (TMD) monolayers. This technique however, has its drawbacks as TMDs tend to grown in random locations on the insulator surface. Ultimately, for scalable production of devices, it is critical to be able to control the location of MoS2 growth in a bottom-up process directly onto a substrate of interest. This is mainly attributed to controlling nucleation of MoS2 on bare SiO2 substrates which has been noted as a rare and complicated process.
While strategies have been developed to enhance the location and growth of MoS2 over the entire substrate, including oxygen plasma treatment, etched features to create fined scratches and the deposition of organic seed promoters, they lack the ability to provide high-resolution spatial control of the TMD growth. For example Han, G. H. et al, “Seeded growth of highly crystalling molybdenum disulphide monolayers at controlled locations” disclosed a process involving complex, multi-step lithography and delicate materials manipulation that yielded patterned flakes of MoS2. This process is cumbersome and time consuming which does not lend to scale-up processes.
Likewise, Najmaei et al, U.S. Patent Publication No. 20140251204 discloses growth methods for controlled large-area fabrication of high-quality graphene analogs by patterning a surface using photolithography and electron beam deposition process. Electron beam deposition requires bombarding metal-organic gas molecules with an electron beam to dissociate and deposit the metal onto the substrate surface while photolithography uses chemicals to pattern the surface with raised “pillars.” The drawback with these patterning processes is that the deposited patterns create raised “pillars” on the surface of the substrate that can limit its use in downstream device fabrication.
The invention herein addresses these deficiencies by using a simple, one-step surface modification process that allows for nucleation seeding of TMD species and production of thin layer, two-dimensional TMD at precise locations on a substrate without the use of toxic lithography chemicals.